Norman O. Birge

Current-driven magnetic excitations in permalloy-based multilayer nanopillars

We study current-driven magnetization switching in nanofabricated Ni(84)Fe(16)/Cu/Ni(84)Fe16 trilayers at 295 and 4.2 K. The shape of the hysteretic switching diagram at low magnetic field changes with temperature. The reversible behavior at higher fields involves two phenomena, a threshold current for magnetic excitations closely correlated with the switching current, and a peak in differential resistance characterized by telegraph noise, with an average period that decreases exponentially with current and shifts with temperature. We interpret both static and dynamic results at 295 and 4.2 K in terms of thermal activation over a potential barrier, with a current-dependent effective magnetic temperature.

Observation of spin-triplet superconductivity in co-based josephson junctions

We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting-ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.

Dephasing of electrons in mesoscopic metal wires

We have extracted the phase coherence time t f of electronic quasiparticles from the low field magnetoresistance of weakly disordered wires made of silver, copper, and gold. In samples fabricated using our purest silver and gold sources, t f increases as T 22/3 when the temperature T is reduced, as predicted by the theory of electron‐electron interactions in diffusive wires. In contrast, samples made of a silver source material of lesser purity or of copper exhibit an apparent saturation of t f starting between 0.1 and 1 K down to our base temperature of 40 mK. By implanting manganese impurities in silver wires, we show that even a minute concentration of magnetic impurities having a small Kondo temperature can lead to a quasisaturation of t f over a broad temperature range, while the resistance increase expected from the Kondo effect remains hidden by a large background. We also measured the conductance of Aharonov‐Bohm rings fabricated using a very pure copper source and found that the amplitude of the h/e conductance oscillations increases strongly with magnetic field. This set of experiments suggests that the frequently observed ‘‘saturation’’ of t f in weakly disordered metallic thin films can be attributed to spin‐flip scattering from extremely dilute magnetic impurities, at a level undetectable by other means.

Magnetization-dependent Tc shift in ferromagnet/superconductor/ferromagnet trilayers with a strong ferromagnet

We have measured the superconducting transition temperature

Switching current versus magnetoresistance in magnetic multilayer nanopillars

T_c

Specific heat spectroscopy: Origins, status and applications of the 3ω method

of Ni/Nb/Ni trilayers when the magnetizations of the two outer Ni layers are parallel (P) and antiparallel (AP). The largest difference in

Critical current behavior in Josephson junctions with the weak ferromagnet PdNi

T_c

Nonequilibrium tunneling spectroscopy in carbon nanotubes.

occurs when the Nb thickness is just above the critical thickness at which superconductivity disappears completely. We have observed a difference in

Dephasing by extremely dilute magnetic impurities revealed by Aharonov-Bohm oscillations.

T_c

Use of Pd–Fe and Ni–Fe–Nb as Soft Magnetic Layers in Ferromagnetic Josephson Junctions for Nonvolatile Cryogenic Memory

between the P and AP states as large as 41 mK - a significant increase over earlier results in samples with higher